The present invention relates to a photoresist article having enhanced image contrast.
It is well known in the art to produce conventional photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer is not. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a chemical change and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution, while the unexposed areas are largely unaffected, thus producing a so-called positive relief pattern on the substrate.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of very large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming the same photolithographic techniques are utilized, by increasing the resolution capabilities of the resist.
Development contrast refers to the slope of a linear plot of film thickness remaining after various exposure doses versus the log of the exposure dose.
Resist resolution typically refers to the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure which can be reproduced with a high degree of image edge acuity in the exposed and developed photoresist.
In order to produce integrated circuits on a VLSI basis finer geometries and improved materials are required. The method of the present invention relates to the use of a photosensitive, portable, conformable built-on mask on top of a conventional photoresist. It is well known that diffraction of the light used to expose the resist as it passes through the mask and into the resist plays a very important role in decreasing the resolution and contrast that can be achieved with a given resist. This becomes more and more crucial as the circuit element size to be printed decreases. European Patent publication No. 0110165 reveals that contrast can be greatly improved by spinning a layer of a photosensitive liquid on top of a baked resist. The photosensitive layer contains a photobleachable dye which photodecomposes at the same wavelengths or wavelength of light used to expose the underlying resist. Effectively, the upper layer improves the resolution of the aerial image, thereby improving the contrast of the bottom resist. The upper photosensitive layer therefore contains a photobleachable dye that absorbs light used to expose the resist such that after bleaching of the material, the exposed areas of the material should be as transparent as possible, so that the light can efficiently decompose the sensitizer of the underlying photoresist. The rate of bleaching of the dye in the material should be equivalent to or slower than that of the sensitizer in the photoresist. Also, the optical density of the film should be as high as possible.
European Patent Publication 0110165 teaches the use of an arylnitrone as the photobleachable dye with a polymeric binder that is soluble only in environmentally pollutive organic solvents. A solution to the environmental problem was thereafter proposed by L. F. Halle, A Water Soluble Contrast Enhancement Layer, Journal of Vacuum Science Technology, Vol. B3(1), Jan./Feb. 1985. The proposal was to employ a contrast enhancement layer of diphenylamine-p-diazonium sulfate, polyvinyl alcohol, water, and maleic acid. Similarly, European Patent Publication 0161660 uses diazonium salts, binding resin, water and an acid component which enhances the solubility of the diazonium salt in the contrast enhancing layer and prevents the coupling of the diazonium salts with the phenolic component in the film.
However, these prior water based diazonium salt systems need the addition of an acidic compound to both enhance the solubility of the otherwise poorly soluble diazonium salts and also to prevent any coupling of the diazonium salt. In order to enhance the solubility of the diazonium salt in the EP 0161660 solution, an almost equimolar amount of, for example, p-toluene sulfonic acid, needs to be present. The instant invention provides enhanced contrast by providing a mask which is built onto the photoresist layer. In practice, a standard positive working photoresist composition is first coated onto a wafer and dried. On top of the resist is coated a composition which contains a sulfonic acid polymer, a diazonium salt and a coupler in a water solution. The sulfonic acid polymer acts as both a binder and a stabilizer for the solution. Upon imagewise irradiation the diazonium salt photodecomposes with bleaching. In the unexposed areas, if the top layer is passed over alkaline vapor, NH.sub.3, ammonia being preferred, the unexposed diazonium salt and coupler react to form an essentially non-photobleachable dye. The whole system is then flood exposed with any u.v. radiation where the nonphotobleachable dye absorbs; the top layer acts as a portable conformable mask; and the bottom resist is developed out.